The DPP said: "I have concluded that there is a reasonable prospect of conviction and that the test for prosecution is met.

"I confirm that we intend to use assisting offender Gary Hagarty as a witness in this prosecution."

Haggarty, 45, pleaded guilty in the summer to 202 terror offences, including five murders.

He is due to be sentenced and was expected to walk free, having already served three years in custody on remand - the equivalent of a six-year sentence - under the controversial assisting offender legislation.

A contentious state deal offered him a significantly reduced prison term in exchange for his evidence against other loyalist paramilitaries.

This new charge in relation to the Fox and Convie killings would be first time his evidence has been used against another person.

Haggarty's case is due to return to court on Wednesday, when legal submissions will be made.

Mr McGrory said another two suspects reported in relation to the murder of John Harbinson in 1997 using Haggarty's evidence would not be prosecuted because the test of a reasonable prospect of conviction had not been met.

He added: "I have spoken before about the complex task of assessing the credibility of an assisting offender.

"In relation to Gary Haggarty, given all the relevant circumstances including his admitted criminality, I consider that his evidence alone is insufficient to prove an allegation beyond a reasonable doubt.

"The evidence currently available to us does not provide the necessary support for Haggarty's identification of the reported suspects as those involved in the abduction and murder of Mr Harbinson."